Cincinnati supports ManifestList images since https://github.com/openshift/cincinnati/pull/711

This support does not exist in the dkrv2 plugin of Cincinnati and needs to be added to support multi-arch graph-data images that are created using oc-mirror. 

Just like the above-linked PR, we need to check if the image is ManifestList. If it is a manifest list image, download any image to extract its contents. 

This is only needed as long as we need to support executable graph-data container images (old form and we want to eventually get rid of this). Eventually we want graph-data images to be FROM scratch and not be executable (OTA-875)

Definition of Done: 

• dkrv2 plugin supports manifestlist images.